Plasticized Cellulosic Films by Partial Esterification and Welding in

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Plasticized cellulosic films by partial esterification and welding in low-concentration ionic liquid electrolyte Xun Niu, Yating Liu, Alistair William Thomas King, Sami Hietala, Orlando J. Rojas, and Hui Pan Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.9b00325 • Publication Date (Web): 15 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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Plasticized cellulosic films by partial esterification and welding in low-concentration ionic liquid electrolyte Xun Niu †,‡ Yating Liu † Alistair W. T. King,& Sami Hietala,& Orlando J. Rojas,‡,* and Hui Pan†,*

†

Jiangsu Co-Innovation Center for Efficient Processing and Utilization of Forest Resources,

College of Chemical Engineering, Nanjing Forestry University, 159# Longpan Road, Nanjing 210037, PR China. ‡

Department of Bioproducts and Biosystems, School of Chemical Engineering, Aalto University, P.O. Box 16300, FIN-00076 Aalto, Espoo, Finland.

& Materials

Chemistry, Department of Chemistry, Faculty of Science, University of Helsinki, A.I. Virtasen aukio 1, P.O. Box 55, FIN-00014, Finland.

*

To whom correspondence should be addressed. Emails: [email protected] (Hui Pan);

[email protected] (Orlando J. Rojas).

ABSTRACT Alternatives to petroleum-based plastics are of great significance not only from the point of view of their scientific and practical impact but to reduce the environmental footprint. Inspired 1 ACS Paragon Plus Environment

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by the composition and structure of wood’s cell walls, we used phenolic acids to endow cellulosic fibers with new properties. The fiber dissolution and homogeneous modification were performed

with

a

recyclable

ionic

liquid

(IL)

(tetrabutylammonium

acetate

([N4444][OAc]):dimethyl sulfoxide) to attain different levels of reaction activity for three phenolic acids (p-Hydroxybenzoic acid, vanillic acid, and syringic acid). The successful autocatalytic Fischer esterification reaction was thoroughly investigated by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, elemental analysis and nuclear magnetic

resonance

spectroscopy

(13C

CP-MAS,

diffusion-edited

1H

NMR

and

multiplicity-edited HSQC). Control of the properties of cellulose in the dispersed state, welding, and IL plasticization were achieved during casting and recrystallization to the cellulose II crystalline allomorph (XRD). Films of cellulose carrying grafted acids were characterized with respect to properties relevant to packaging materials. Most notably, despite the low degree of esterification (DS 90%) and water resistance (WCA ~ 90°). Moreover, the measured water vapor barrier was found to be similar to that of PLA composite films. Overall, the results contribute to the development of the next generation green, renewable, and biodegradable films for packaging applications. Keywords: cellulose, phenolic acids, ionic liquid, welding, packaging.

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INTRODUCTION Concerns regarding the pollution generated by plastics used for packaging have led to increased efforts to find low-cost alternatives, preferably if they display multiple properties such as sustainability, flexibility, transparency and strength, as demanded by the given applications. It is not surprising that cellulose has attracted renewed attention, given its possibility for chemical modification and for developing features that are desirable in biomedical,1 electronic,2 and optical3 applications. Cellulose can also be bestowed with adsorption and separation capabilities,4-5 hydrophobicity,6 and stimuli responsiveness.7-8 However, modification methods such as esterification, silanization, and etherification are often difficult to implement and, especially, to scale up, due to the inherent use of hazardous chemicals, solvents and release of toxic gases.9 The abundant hydroxyl groups on the surface of cellulosic materials engage in intermolecular and intramolecular hydrogen bonding, which result in their insolubility in common solvents and difficult to process. Thus, it is desirable to process cellulose via simple, efficient, and environmentally friendly methods. In this context, the conversion of cellulose into homogeneous solutions is quite attractive and the key for this process is to find efficient, low-toxicity, chemically stable and recyclable cellulose solvents. Some Ionic liquids (ILs), often composed of mixed organic and inorganic anions and cations, have been investigated for fractionation of lignocellulosic materials and are able to dissolve cellulose and its derivatives.10 Compared to conventional solvents, ILs are often considered as “green solvents” due to their low volatility and the ability of some ILs to dissolve cellulose. 3 ACS Paragon Plus Environment


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However, they should only be considered so if they have high chemical, and thermal stability, low toxicity and are recyclable. The high anion basicity required to disrupt the hydrogen bonding network in crystalline cellulose introduces instability to an ionic liquid,11 therefore, careful choice of ionic components is important. Despite, their unique properties, the high cost of typical ILs and challenges in their recovery severely limit their use. Thus, efforts have been made to investigate co-solvents to reduce the overall cost. Dimethyl sulfoxide (DMSO), for example, has been reported to improve cellulose swelling and solubility and to disrupt hydrogen bonding. Co-solvents as such may also play roles as “viscosity reducers” and enable faster mass transport of the system with little or no interaction with the given ILs.12 Thus, there are advantages to finding as low consistency mixtures as possible, yet with the ability still to mould or modify the properties of cellulose. ILs also allow for further chemical conversions, not possible in heterogeneous systems. In this respect, phenolic acids having one or more phenolic hydroxyl groups on the aromatic ring can be considered as suitable bio-based feedstocks for chemical modifications. They can be derived from plants (including fruits, vegetables, and mushrooms) or from lignin biodegradation.13-14 These compounds include vanillic acid (VA), p-hydroxybenzoic acid (HA), and syringic acid (SA), which are attractive owing to additional beneficial properties, including anti-oxidant, anti-inflammatory, antitumor, and hypoglycemic activities.15 The primary and the secondary wall of plant cells include a hierarchical assembly of cellulose microfibrils, hemicelluloses and aromatic lignin bound together by chemical, physical and other links,14 all of 4 ACS Paragon Plus Environment

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which contribute to a flexible yet strong structure. Inspired by related principles, biomimetic materials can be developed. Recently, phenolic acids have been applied to nanoparticles16 and polymers17-18. Especially, VA has been used for grafting onto chitosan to develop antioxidant activity, for instance for packaging of fish oil;18-20 while amides, formed from reaction of chitosan with phenolic acids, are expected to form more readily on cellulose than oxo-esters, this provides a motivation to consider other phenolic-grafted, bio-based polymers to achieve bioactive packaging and food additives. Typical cellulose-based packaging materials are reported to have a tensile strength of 100−300 MPa and strain of 1−10%.21 Despite this outstanding performance, such materials and, especially their brittleness, still need improvement to become alternatives to plastics. Cellulose-based films that simultaneously display high flexibility, strength and transparency are still to be developed and their potential utilization as plastic remain as a challenge. Herein, inspired by the nature of plant’s cell walls, we consider the mechanical and functional performance of packaging materials with regards to their intrinsic properties and interfacial adhesion. A series of cellulose esters grafted with phenolic acids (HA, VA and SA) were prepared using a room-temperature ionic liquid system, tetrabutylammonium acetate ([N4444][OAc]): dimethyl sulfoxide (DMSO). The phenolic acids feature a different number of methoxy groups (-OCH3), from 0, 1, to 2 in HA, VA and SA, respectively. Accordingly, they display variable reactivity with cellulose, affecting its performance after grafting. The influence of the structural characteristics of the grafted moieties (HA, SA and VA) and chain segments of 5 ACS Paragon Plus Environment


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the cellulose were studied as far as changes in structure, elemental composition and bonding after modification. After removal of any residual acids and ILs, the given systems were cast into all-cellulose films. Most notably, ILs and excess acids were easily recovered and recycled after gelling and regeneration cellulose through a simple purification step. The properties of the three types of acids grafted on the cellulose films were compared with respect to morphology, crystallinity, transmittance, hydrophobicity, and processability. The IL-mediated modification with phenolic acids of cellulose allowed new types of films that favorably compete with petroleum-based plastics aimed for packaging.

EXPERIMENTAL Materials. Unrefined, bleached softwood pulp board were kindly supplied by the Institute of Chemical Industry of Forestry Products, Chinese Academy of Forestry and used as the cellulose precursor material. The pulp board was first cut into small segments and ground to obtain a loose powder before its dissolution. [N4444][OAc] was purchased from Sigma-Aldrich (Santa Clara, USA). p-Hydroxybenzoic acid (HA), vanillic acid (VA), and syringic acid (SA) were purchased from Alfa Aesar (Karlsruhe, Germany). Acetone and dimethyl sulfoxide were obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Other chemicals and solvents were of analytical reagent grade. All chemicals were used without further purification. Homogenous-phase modification of cellulose. The [N4444][OAc]/DMSO IL system was prepared according to our previously reported procedure.22 In short, 10 mL of 6 ACS Paragon Plus Environment

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[N4444][OAc]/DMSO (8:92 wt%) was used to obtain a 4 wt % cellulose solution upon stirring at room temperature for 30 min. Concurrently, in a separate flask, 0.2 g of either HA, VA or SA, applied in solid form, was dissolved in 5 mL DMSO. The respective acid solution was added to the cellulose solution, thereby lowering its viscosity, which facilitated homogenous reaction in an oil bath at 120 °C and under stirring at 600 rpm. The reaction was stopped after 3 hours by centrifugation at 12,500 rpm with DMSO to remove some undissolved and impurities. Cellulose films. The protocol used to homogeneously modify cellulose and prepare the respective films is shown in Figure 1. 15 ml of the respective solution was cast on Petri dishes (polystyrene; 100 mm diameter) and dried in an oven at 70 °C for 12 h, which allowed gelation (yielding gel suspensions), involving the partial dissolution and swelling of cellulose. Then the films were formed by the regeneration of dissolved cellulose at room temperature followed by multiple washing with acetone (nonsolvent) to remove residual acid. The films were then immersed in a large amount of water, which was exchanged every day for 1 week until the respective hydrogel film achieved equilibrium. The gel films were then dried in ambient air for 72 h. In addition, films of neat cellulose were prepared as the reference using a similar procedure: A total of 4 wt % of same cellulose material was submerged in 10 mL of 8 wt% [N4444][OAc]/DMSO solution and stirred at room temperature for 30 minutes. Concurrently, 5 mL DMSO was added to cellulose [N4444][OAc]/DMSO solution, the mixture was then placed in an oil bath at 120 °C and stirred at 600 rpm for 3 h. After oven drying at 70 °C for 12 h, the pure cellulose films were washed with acetone and water, dried again, and labeled as CEL. 7 ACS Paragon Plus Environment


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Accordingly, the respective dried films obtained after homogeneous modification of cellulose film samples are therein referred to as H-CEL, S-CEL, and V-CEL, corresponding to modification with HA, SA, and VA, respectively. IL, solvent and acid recovery. The washing fluids, water, acetone, unreacted acids, [N4444][OAc], and DMSO were subjected to recovery. Acetone was firstly evaporated using the rotary evaporator, then the remaining effluent was oven-dried to remove water and residual DMSO. The solid phase comprised the respective acid and [N4444][OAc]. The collected acids, [N4444][OAc], and acetone can be reused for several cycles to enable re-processing (reaction and washing) as shown in Figure 1. Morphological Analysis. The cross-sections of the prepared films were observed under a JSM-7600F SEM (JEOL, Japan). The films were frozen and then fractured in liquid nitrogen. The cross-sections of the fractured films were mounted on aluminum stubs with carbon tape and coated with gold for 1 min with 5-10 nm thick and then imaged with an accelerating voltage of 3-5 kV. X-ray Photoelectron Spectroscopy and elemental analysis. Surface composition of pure CEL and modified cellulose (H-CEL, V-CEL, and S-CEL) were recorded with monochromatic A1 Kα irradiation at 100 W and effectively charged to neutralization with slow thermal electrons, using AXIS Ultra instrument (Kyoto, Japan). C−C/C−H assigned to the C1s signal was set at 285.0 keV. XPS PEAK 41 program was used for the spectrum decomposition for C1s, with the subtraction of a Shirley background before Gaussian functions. The degree of surface 8 ACS Paragon Plus Environment

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substitution (DSS) was calculated from Eq. (1):23 DSS = C (O-C=O)/ [C cellulose/6)]

(1)

where C(O-C=O) is the percentage of O-C=O groups from the grafted acids and Ccellulose is the percentage of the C-O and O-C-O groups. The DS in the bulk was also calculated from elemental analysis by using a Vario Macro cube elemental analyzer (Elementar, Fulda, German) which was used to determine the carbon, hydrogen, nitrogen, and oxygen content in the unmodified and modified celluloses (H-CEL, V-CEL, and S-CEL). Nuclear Magnetic Resonance (NMR). Liquid-state NMR spectra were acquired on a Bruker AVANCE NEO 600 MHz spectrometer equipped with a 5 mm SmartProbeTM. The samples were dissolved in the ionic liquid electrolyte, tetrabutylphosphonium acetate ([P4444][OAc]):DMSO-d6 (20:80 wt%), according to a published procedure.24 However, the final 5 wt% viscosity was too high, due to the high molecular weight of the materials, preventing transfer to the 5 mm NMR tube and likely offering poor resolution. This is typical of Kraft pulps. Therefore, the sample was diluted with further [P4444][OAc]:DMSO-d6 electrolyte to a concentration of ~3 wt%. 1H and diffusion-edited

23 1H

experiments were collected for CEL,

H-CEL, V-CEL and S-CEL. The diffusion-edited 1H was used to filter out the low molecular weight species in the sample, to allow for identification of the polymeric species that may not be visible under the H2O or [P4444][OAc] signals. A multiplicity-edited HSQC for S-CEL was collected to help identify the cellulosic resonances and those of other potential impurities. Full 9 ACS Paragon Plus Environment


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details of the NMR experimental conditions and pulse sequences are given in the supporting information, which also includes measurements via

13C

CP (Cross-Polarisation) MAS

(Magic-Angle ndSpinning) NMR. Water Contact Angle. The water contact angle (WCA) was determined using an optical contact angle meter SL 100B from Solon Information Technology Co., Ltd. (Shanghai, China) at a RH of 50% and 23 °C. To compare different samples, each contact angle was taken at 60 s, and the average value of at least three measurements is presented. Barrier Properties. Water vapor sorption (WVP) of the films was measured by a Bei Shide 3H-2000PW dynamic vapor sorption analyzer. The temperature and the relative humidity (RH) were set at 25 °C and 50%, respectively, to obtain the equilibrium adsorption capacity. At least three samples were characterized. Water Absorption. Film water absorption was determined by placing circular film (4 cm diameter) into 10-cm diameter Petri dishes containing 30 ml deionized water. The weight of the film before and after 30 min immersion was obtained to determine the amount of water absorbed. The water absorption rate was calculated based on the change in weight with time. Mechanical properties. The mechanical properties, including the tensile stress, tensile strain, and Young’s modulus of the films were measured using a mechanical testing unit (Shenzhen, China). 40×10 mm2 specimens were cut at random positions on the films and were conditioned for 1 day at 23oC and 50% RH. A gauge length of 10 mm, 500 N loading cell, and a crosshead speed of 5 mm/min were used. The thickness of the specimens was measured at five 10 ACS Paragon Plus Environment

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random locations for each film (NSK micrometer, Japan). At least ten samples of each film were tested, and the average values are reported with the standard deviation. X-ray Diffraction. X-ray Diffraction (XRD) patterns were obtained at room temperature using an Ultima IV X-ray diffractometer (RigaKu, Tokyo, Japan) with Cu Kα, at a 40 kV acceleration voltage and a 30 mA current. The diffraction was recorded at a scan rate of 5° min−1, in the range of 2 = 5−30°. Transmittance. The transparency of the films was measured using a UV-vis spectrophotometer (Shimadzu UV-240, Japan). The transmittance spectra of the samples over the 400–800 nm range was recorded. Film specimens were cut into rectangular shapes and placed in an integrating sphere; three duplicates of each sample were analyzed and transmittance was normalized by the respective thickness of the film, to account for the small differences in this parameter.



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Figure 1. Preparation of the cellulose films modified with phenolic acid (HA, VA, or SA), using IL electrolyte as the solvent. Also shown is the process used to recycle the ILs, acids, and acetone.

RESULTS AND DISCUSSION Morphology of CEL and modified cellulose films. Typical CEL films show a layered structure formed by randomly oriented fibrils, as revealed by cross-section images (SEM), Figure 2a. Clear features are observed with sizes in the micro- and nano-meter scales. Compared to the film of unmodified cellulose, CEL (35 m), the samples treated with acids (Figure 2b, 2c 12 ACS Paragon Plus Environment

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and 2d), were denser according to the following order: H-CEL (33 m) V-CEL>S-CEL. In the case of the S-CEL films (1069 g·μm/m2·day·kPa), a high-water barrier at 50% RH was also observed. The water vapor permeability measured at 23 °C and 50% RH of the acid-grafted cellulose films was slightly higher than those commercial polymers and nanocellulose-reinforced biodegradable polymer films (for example, PLA: 898 g·μm/m2·day·kPa38 and polyhydroxyalkanoate, PHA: 824 g·μm/m2·day·kPa

39).

Moreover, the barrier performance of V-CEL and S-CEL films was

slightly better than that measured for composite films of PLA reinforced with 15% cellulose nanocrystals (CNC) (1901 g·μm/m2·day·kPa)40 as well as polycaprolactone (PCL) reinforced with 5% CNC (1510 g·μm/m2·day·kPa) 41.



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Figure 6. Water contact angle (images shown in the inset) and degree of surface substitution (DSs) obtained by XPS for CEL, H-CEL, V-CEL and S-CEL films.


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Table 1. Density, porosity, water absorption and water vapor permeability of CEL, H-CEL, V-CEL and S-CEL films. Film

Density (g·cm3)

Porosity（%）

CEL H-CEL V-CEL S-CEL

0.92 0.95 1.02 1.09

31.3% 29.1% 23.8% 15.8%

Water absorption (%) 84.1 66.2 56.6 50.8

WVP (g·μm/m2·day·kPa) 2881 1689 1240 1069

Mechanical properties. Typical tensile profiles and the average value of elongation at break and Young’s modulus are presented in Figure 7. For CEL films the tensile strength, modulus, and strain at failure were 99 MPa, 3.4 GPa and 3.4%, respectively, which are characteristic of a fragile material. In contrast, the modification of CEL improved the toughness, with a significant improvement in ductility for H-CEL, V-CEL, and S-CEL (the increased strain of 571%, 760% and 808%, respectively). Moreover, the acid treatment of CEL resulted in only a slight decrease in strength, from 100MPa to 80-90 MPa. The increased strength accompanied by a simultaneous improvement in toughness is ranked according to H-CEL < V-CEL < S-CEL. The relatively high strength of V-CEL and S-CEL films is explained by the high density of grafted, rigid phenolic structures (1, 2 -OCH3 on the benzene ring) and the -OH from phenolic acids that contribute with hydrogen bonding with cellulose. Most distinctive, the trend implies an increased stiffness with a simultaneously more stretchable film, which is rarely reported for nanofiller-reinforced composites. After modification in ILs, the modified CEL films displayed a slight decrease in Young’s 23 ACS Paragon Plus Environment


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modulus compared to that of CEL, which is probably associated with the change in the crystalline structure caused by the dissolving-regeneration process and the effect of excess acids. Possible explanations for this observation are that the ILs, known as efficient solvents of cellulose, could swell the compact structure of cellulose fiber and may have converted micro-sized fibers or fiber clusters into nano-sized fibrils, which resulted in the reduction in crystallinity or conversion to cellulose II, and simultaneously increased surface area for stress transfer.42 Additionally, the IL could work as a welding agent,43 creating disordered cellulose domains on the surface of the nano-sized fibrils, connecting them together and forming a continuous and integrated network. During the welding process, the bulk and surface of the cellulose was likely plasticized by ILs and became more flexible. Moreover, as can be seen in Table 1, the density of the films increased from the original 0.92 to 0.95, 1.02 and 1.09 g/cm3 for CEL, H-CEL, V-CEL and S-CEL, respectively. The film with the highest density was S-CEL, possibly indicative of increased reactivity in the presence of syringic acid.


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Figure 7. Tensile strength of CEL, H-CEL, V-CEL and S-CEL films: tensile stress−strain curves (a) and measured Young’s modulus and elongation at break (b). Error bars correspond to the calculated standard deviation. Film Crystallinity. Wide angle X-ray diffraction patterns of the raw fibers and modified CEL films clearly show changes in crystallinity (Figure 8). The CEL film shows the presence of 25 ACS Paragon Plus Environment


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native cellulose I crystalline allomorph. In contrast, H-CEL, V-CEL, and S-CEL films showed a clear reduction in crystallinity and recrystallization to the cellulose II crystalline allomorph.44 Small diffraction peaks of (1 1 0) and (2 0 0) from the native cellulose I allomorph were detected in the XRD patterns of the acid-modified cellulose films. It is noteworthy that the V-CEL showed the largest degree of cellulose II crystallinity, then the S-CEL, and H-CEL. These results indicate increased crystalline components in the films with increasing aspect ratios of the crystal of cellulose II,45 which explains the observed improvement in stiffness of the acid-modified cellulose films.

Figure 8. Wide-angle X-ray diffraction spectrogram of CEL, H-CEL, V-CEL and S-CEL films.

Light Transmittance. UV/Vis spectroscopy confirmed a high light transmittance for all 26 ACS Paragon Plus Environment

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films, over the entire range of visible and near-infrared light (Figure 9). Neat CEL films prepared in [N4444][OAc]/DMSO system (average thickness of 30 μm), for example, had a light transmittance of 88% at 550 nm, which is higher than most of the other all-cellulose composite films containing native or regenerated cellulose fibers. The transparency of all-cellulose films have been reported to be ~80% (170 μm thickness),46 >80% (140 μm-thick films of NaOH-treated cellulose fibers),47 up to 88% (30-μm thick TEMPO-oxidized cellulose nanofiber films)48 and 85% for films obtained from regenerated cellulose.49

The higher transparency of

the modified cellulose films indicates a highly uniform structure. The porosity (reduced porosity of the modified cellulose films), from 31% in CEL to 15% in S-CEL, correlate with a reduced light scattering at the fibril interfaces and the contribution of voids.50

Figure 9. Light transmittance (at visible range) results of CEL, H-CEL, V-CEL and S-CEL 27 ACS Paragon Plus Environment


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films.

Chemical recovery. Chemical recovery is important under the concept of green chemistry. Based on our previous work, [N4444][OAc]/DMSO is an effective solvent for dissolving cellulose and it is relatively stable during processing.22 Therefore, the reusability of [N4444][OAc] and the acids was assessed upon washing of the modified CEL films with an organic solvent (acetone). The recovery procedure, shown in Figure 1, included acetone direct distillation (rotary evaporation), while the crude acids and [N4444][OAc] were recovered by oven-drying the residuals. The solid phase comprised the respective acid and [N4444][OAc], which were extracted with dichloromethane after washing several times with distilled water for further purification. It was found that the recovery rate of [N4444][OAc] was >90% (for a purity of ~ 40%). The synthesis of new films was demonstrated by recycling the recovered chemicals, indicating the promise of [N4444][OAc] and the used phenolic acids for such purpose. Naturally, the efficiency of the system diminished with the number of cycles.

CONCLUSIONS Inspired by the composition and structure of plant’s cell wall, cellulosic fibers were modified with phenolic acids in low-concentration IL electrolyte systems under homogeneous conditions. Extended gelation and regeneration resulted in hydrophobic films. Despite the low degree of esterification, the films displayed a remarkable mechanical performance and optical transparency. No additional catalyst was used to enable the Fischer esterification reaction 28 ACS Paragon Plus Environment

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indicating the reaction was autocatalytic. This is beneficial for the recyclability of the system as components are kept to a minimum, which requires only some solvent evaporation and acid makeup. The products were characterized by FT-IR, XPS, elemental analyses, liquid-state NMR and solid-state NMR to understand the reactivity. Highly transparent films (H-CEL, V-CEL, and S-CEL) with a thickness of 30 μm were obtained easily by casting. The acid-modified cellulose films possessed excellent mechanical strength, with a Young’s modulus of 2-3.5 GPa and a strain at break of up to 35%. The films were flexible and tough, superior to those of paper and most other reported all-cellulose films. The excellent mechanical properties achieved owes to the synergistic effect of esterification, morphology and porosity changes (electrolyte ‘welding’ effect). NMR vs XPS suggests a possible enrichment of ester at the film surface. In addition, the films showed good hydrophobicity and barrier properties, highly favorable for making them a promising alternative as bio-based plastics and for application in packaging materials, optical and electronic devices.

Supporting Information Description of FT-IR and elemental analysis methods; table of elemental content in films; FT-IR spectra; 1H NMR spectra of bulk CEL, H-CEL, and V-CEL films, and 13C CPMAS NMR spectra of S-CEL films.

ACKNOWLEDGEMENTS H.P., X.N. and Y.L. are grateful for the financial support by the Forestry Industry Research 29 ACS Paragon Plus Environment


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Special Funds for Public Welfare Projects (201504602), Nature Science Foundation of China (31770631) and the Priority Academic Program Development (PAPD) project of Jiangsu Higher Education Institutions. O.J.R. and X.N. acknowledge funding support by European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (ERC Advanced Grant agreement No 788489, “BioElCell”). A.W.T.K. acknowledges funding support from the Finnish Academy under the project ‘WTF-Click-Nano’ (311255).

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